Emerging chromatographic techniques such as high performance capillary electrophoresis, packed capillary liquid chromatography, and super critical chromatography require spectroscopic measurements to be made on extremely small volumes of flowing liquid samples. The typical application has a sample flowing through a fused silica capillary tube with inside diameters ranging from fifteen to one hundred-fifty micrometers and with outside diameters of one hundred-fifty to three hundred micrometers. Various techniques presently are used for directing light from a suitable source into and/or through such a small volume sample cell, as well as taking the light emanating from the inside of the cell and directing it toward a light detecting or analyzing instrument to effect optical analysis or detection of samples contained in the cell. Alignment of the optical system so as to appropriately direct the light from the source to the capillary cell and particularly to the bore and sample therein and/or direct the radiation emanating from the cell to a detector has been a constant problem.
The underlying problem can be generally described as one of directing light from a light source of finite extent to or through a volume of interest that has a cross sectional area perpendicular to the optical axis which is less than that of the source, or directing light that emanates from or passes through a volume of interest to a photodetector or analyzer that has a cross sectional area larger than that of the volume of interest. This is achieved by using a lens or system of lenses to image the source into the volume of interest at a magnification that matches the size of the image to the cross sectional size of the volume of interest or to similarly image the volume of interest onto a photodetector or analyzer. The numerical aperture of the system as viewed from the volume of interest will be greater than the numerical aperture as viewed from the source by a factor of one divided by the magnification. It is generally desired to transfer the greatest possible amount of light from the source to the image by using a low f/number lens system.